1. Field of the Invention
This invention relates to the field of synthetic cable terminations. More specifically, the invention comprises a method for terminating a multi-stranded cable having at least a partially non-parallel construction.
2. Description of the Related Art
Synthetic rope/cable materials have become much more common in recent years. These materials have the potential to replace many traditional wire rope assemblies. However, the unique attributes of the synthetic materials can—in some circumstances—make direct replacement difficult. The smallest monolithic component of a synthetic cable will be referred to as a filament. Bundles of such filaments will be referred to as a “strand.” Strands are then gathered to make a cable. In some instances strands will be grouped into “strand groups,” and these strand groups will then be gathered to make a cable.
A synthetic filament is analogous to a single wire in a bundled wire rope. However, in comparison to the relatively stiff steel wire used in a wire rope, the synthetic filament: (1) is significantly smaller in diameter; (2) is significantly less stiff (having very little resistance to buckling); (3) has a much lower coefficient of friction.
These differences become particularly significant when dealing with a multi-stranded cable having a non-parallel construction. A discussion of the prior art will illustrate this point. FIG. 1 shows a prior art cable 10 constructed by helically winding six exterior strands 12 around a single core strand. This is a partially non-parallel construction. The single core strand runs parallel to the cable's central axis. However, the six exterior strands form a helical path and are clearly not parallel to the central axis. Such a cable may generally be referred to as having a “non-parallel” construction. A non-parallel cable may have some parallel components (such as a core strand or bundle of strands and possible one or more parallel outer layers). However, a load-bearing portion of its total mass is made of non-parallel strands. These may assume the form of a helix (as shown in FIG. 3), a braid, or any other suitable configuration.
Those skilled in the art will know that a construction such as shown in FIG. 1 does not distribute equal loads in all the strands when the cable is loaded in tension. The helical winding in the outer layer will produce an “unwinding” force as all the strands attempt to straighten under tension. This phenomenon becomes even more complex with three and four layer non-parallel cables. These tend to include overlapping helical layers with alternating directions of twist. Shorter strands tend to receive a relatively larger tensile load than longer strands.
Individual wire strands in a traditional wire rope such as depicted in FIG. 1 have relatively high strand-to-strand friction forces. These forces tend to inhibit the strands slipping over one another. Thus, a wire rope cable tends to retain a stable cross section and tends to distribute tensile loads fairly evenly. Further, the strands do not tend to be displaced longitudinally (along the direction of the cable strand).
FIGS. 2 and 3 illustrate a typical construction for a strand made of synthetic filaments. In FIG. 2, each strand 12 may include a large number of filaments 16 encompassed within an encircling jacket 14. In other instances, the filaments will be twisted or braided together with no external jacket.
In FIG. 3, groups of seven strands 12 are twisted to form seven strand groups 20. These strand groups are then assembled and retained in position by a much larger encircling jacket 14 (which may be an extruded polymer, a braided outer layer of strands, or even a “whipping” of a single strand wrapped helically around the entire cable). Again, a jacket may or may not be included. If the strand groups are twisted or woven then the external jacket may be omitted. The reader will observe that each strand group 20 is actually parallel (the center axis of each strand group runs parallel to the center axis of the cable as a whole). However, within each strand group most of the individual strands are non-parallel.
Cutting, handling, and terminating such cable assemblies present new challenges. Even a relatively large cable 10 such as shown in FIG. 3 has little compressive stiffness along the direction of the center axis. This means that individual filaments and strands can easily slip longitudinally over each other. If the cutting and terminating method does not account for this factor, the completed cable may have wide variations in filament lengths. This problem of course means that the shorter filaments will receive a higher load and will fail prematurely.
FIG. 4 shows a length of cable 10 stored in a coil 22 (The coil is typically formed by winding the cable onto a spool). The cable is typically straightened for processing. Unconstrained bends occurring in this process can cause unwanted filament dislocations. Accordingly, it is important to constrain the cable so that these dislocations may be reduced or eliminated.
Problems also arise when a sheared end is locked into a termination in order to create a termination. If some filaments are longitudinally dislocated during the process leading up to the addition of the termination, then the resulting cable will not have an even load distribution across its filaments.
It would therefore be advantageous to provide a method of cutting and terminating a multi-strand non-parallel cable which would reduce the problems inherent in the use of synthetic filament ropes/cables. The present invention proposes just such a method.